Abstract

For the 32-year period from 1979 to 2010, trends of surface and subsurface temperature and meridional motion of the current system in the Antarctic Circumpolar Current (ACC) region are studied with in situ observations and an eddy-resolving general circulation model. The observed and simulated surface temperature shows a similar pattern between the Atlantic and Pacific: warming to the north of the Subantarctic/Subtropical Fronts in the Atlantic and of the Subtropical Front in the Pacific and cooling to the south of those fronts. The subsurface temperature trend, again from both observation and model, reveals an asymmetric pattern between the Atlantic and Pacific: subsurface warming is dominant over the whole ACC region in the Atlantic, while both warming and cooling are significant in the Pacific, the former located to the north of the Subantarctic Front and the latter to the south. The model reveals that the ACC has generally shifted poleward in the Atlantic, while it has shifted equatorward around Subantarctic Front and Polar Front in the Pacific. The ACC shift is consistent with the overall subsurface temperature trend. The basin-scale difference of the ACC response can be related to the different regime of the trend in meridional gradient of the zonal wind stress to the north and south of 50–55°S and suggests a coupling of the ACC and overlying westerly on the multi-decadal time scale.

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Acknowledgments

We thank Akira Taniguchi for his data handling of OFES. Discussion with Drs. Andy Hogg and Katsuro Katsumata are very helpful in interpreting the results. Comments and suggestions from two anonymous reviewers helped substantially improve this manuscript. This work was supported by Grant-in-Aid for Scientific Research (22106009) of the MEXT of the Japanese Government, by the Cooperative Research Centre program of the Australian Government, through the Antarctic Climate and Ecosystems Cooperative Research Centre and by the Australian Government Department of the Environment, the Bureau of Meteorology and CSIRO through the Australian Climate Change Science Program.

Appendix: Climatological fronts of OFES

The meridional gradient of the climatological mean (1979–2010) SSH reveals lines of maxima which merge and diverge at some longitudes (Fig. 13). Here the climatological the ACC fronts (SAF and PF) in OFES are defined as the climatological SSH contours where maxima in the meridional gradient of climatological SSH in 0.1° horizontal resolution are roughly located. Since large meridional gradients lie around the climatological SSH contours of −50 and −100 cm (0 cm reference is the global mean SSH) in circumpolar average (Fig. 14) and the lines are roughly associated with the local maxima (Fig. A1), the climatological SSH contours of −50 and −100 cm are defined as the SAF and PF, respectively.